An injury to the anterior cruciate ligament can be a debilitating musculoskeletal injury to the knee, seen most often in athletes. Non-contact tears and ruptures are the most common causes of ACL injury. The anterior cruciate ligament is an important ligament for proper movement. ACL injury more commonly causes knee instability that does injury to other knee ligaments. Injuries of the ACL range from mild such as small tears to severe when the ligament is completely torn. There are many ways the ACL can be torn; the ligament tears because it is overstretched. The movements of the knee that can result in a tear is when the knee is straightened more than 10 degrees beyond its normal maximal position - the lower leg is forced forward in relation to the upper leg. It can happen from a rotational injury when the knee twists when the rest of the body is moving straight. Tears in the anterior cruciate ligament often take place when the knee receives a direct impact at the knee while the leg is in a stable position, for example a standing football player is tackled sideways when his feet are firmly planted.